In industries that make use of pipes for the delivery of gas or fluid, valves such as shut-off valves are common components. Such valves generally include a rod connected with the valve body. The rod automatically performs a linear movement in response to a valve shut off due to downstream pressure that is above or below a predefined point. The rod is guided in a rod guide assembly for protection and sealing.
As shown in FIG. 1, a common used rod guide assembly 10 includes a general disc-shaped cover part 11 and a cylinder-shaped body part 12 which has a smaller diameter comparing to the cover part 11. The cover part 11 together with the body part 12 defines a guide channel in which the valve rod 40 slides. The cover part 11 and the body part 12 are formed integrally. The body part 12 can be inserted in a port of a valve casing 50 and the cover part 11 is secured to the casing 50 via a connecting means such as one or more screws 18. As shown in FIG. 1, a retainer 17 is positioned in a lower end of the body part 12 such that a first seal 13 and the back-up rings 15 are supported in a lower recess of the body part 12 to seal the engagement between the rod 40 and the rod guide assembly 10.
The commonly used rod guide assembly 10 possesses a number of drawbacks. For example, considering the strength requirement of the integrated formed rod guide assembly 10, the rod guide assembly 10 may only be configured from a high strength material such as alloy steel. However, such a material usually has a higher friction coefficient with the valve rod 40 (e.g., usually made from steel) which can result in a poor guiding performance.
In the case of the integrated formed rod guide assembly 10, the first seal 13 is held by means of a retainer 17. Due to the dimension of the retainer 17 and therefore a larger lower area of the body part 12 contacting with the fluid, the rod guide assembly 10 is subjected to a significant force Fp from the fluid, which tends to pull the rod guide assembly 10 apart especially in the case of the fluid having a high pressure such as above 10 Mpa (i.e., PED certificate class 600#). Thus, the screws 18 should have a higher strength to assist in maintaining the rod guide assembly 10 in place. A solution to this problem may be to increase the size of the screws 18, which is not allowable in compact circumstances. Another solution may involve selecting material with higher strength for screws 18, but this approach increases the cost.